This invention relates to a joint assembly for joining members comprised of different materials and, in particular, to a joint assembly for joining metallic and ceramic members.
Ceramic members may be joined to metallic members using specialized brazing processes. During a brazing process, the ceramic member, metallic member and brazing filler alloy are all subjected to a temperature at the melting point of the brazing filler alloy. A sufficient time at the brazing temperature is allowed for the brazing filler alloy to melt and flow, thereby wetting the surfaces of both the ceramic and the metallic members, which have been properly conditioned for the brazing process.
The resulting brazed assembly is then cooled below the solidification temperature of the brazing material. The ceramic and metallic members, therefore, become joined physically and chemically to form a ceramic and metallic joint assembly via the solidified brazing material.
For electrical or heat isolation, metal-ceramic brazed joint assemblies are widely used. Applications of these joint assemblies can be found in vacuum systems, gas turbine engines, automobile power units, nuclear reactors and electrochemical systems such as fuel cells. For low temperature applications, such as in vacuum systems, metal-ceramic thermal mismatch is not significant, so that the process is relatively straightforward. However, for high temperature applications, such as in engines and high temperature fuel cells, especially in corrosive environments, special considerations are required to overcome the thermal mismatch and corrosion effects.
Fuel cells are electrochemical devices which convert the chemical energy of reaction efficiently into electricity. Usually, the fuel and oxidant gases are fed continuously through the inlet gas pipelines. The reacted gases exit the fuel cells through the outlet gas pipelines. Both the inlet and outlet gas pipelines are required to be electrically isolated from the fuel cell stacks. In high temperature fuel cells, such as carbonate fuel cells (operating at xcx9c650xc2x0 C.) and solid oxide fuel cells (operating at xcx9c800-1000xc2x0 C.), the electrical isolation is typically provided by ceramic breaks consisting of metal-ceramic pipe joints. The best commercially available ceramic break for this purpose is Kovar (Fexe2x80x94Ni alloy)-Al2O3 pipe brazed by Ag-based filler metal. This typical break can be used in the carbonate fuel cell environment for a certain period of time without integrity and gas leakage degradation.
Various corrosive environments are encountered in high temperature fuel cells. In the carbonate fuel cell, for example, the fuel gas is a high temperature gas mixture of H2, N2, H2O, CO and CO2, and the oxidant gas is a high temperature gas mixture of O2, N2, H2O and CO2. Carbonate vapors are also present. These atmospheres require metallic materials with a high corrosion resistance. For the solid oxide fuel cell, the temperature is even higher (xcx9c1000xc2x0 C.), and the corrosion attack is severer.
Currently there are no commercially available ceramic breaks that can meet the requirements of sufficient corrosion/oxidation resistance and high temperature strength (650-1000xc2x0 C.) for long-term high temperature applications, and well developed techniques to manufacture the desired products.
It is, therefore, an object of the present invention to provide an improved metal-ceramic joint assembly.
It is a further object of the present invention to provide a metal-ceramic joint assembly having high temperature and corrosion endurance over extended periods of time.
It is yet a further object of the present invention to provide a metal-ceramic joint assembly capable of withstanding the environment of high temperature fuel cells, such as carbonate and solid oxide fuel cells, over extended periods of fuel cell operation.
In accordance with the principles of the present invention, the above and other objects are realized in a metal-ceramic joint assembly comprising a ceramic member, a preselected metallic member spaced from the ceramic member and a preselected brazing alloy situated therebetween. More particularly, the metallic member is selected from one or more of an aluminum containing ferritic stainless steel, a high chromium-content ferritic stainless steel and an iron-nickel alloy with a corrosion protection coating. The brazing alloy, in turn, is selected from one or more of an Au-based and Ni-based alloy having a brazing temperature in the range of 950 to 1200xc2x0 C.
In further accord with the invention, the metallic and ceramic members are physically configured so that the metallic member has a conical outwardly tapered sleeve section and the ceramic member has a conical inwardly tapered region which faces and is spaced from conical sleeve of the metallic member. This space receives the brazing alloy. The conical sleeve section and conical region have the same taper which is at an angle in the range of greater than 15 degrees to less than 45 degrees relative to the sleeve axis.
Also, in accord with the invention, the conical sleeve section is provided with a nickel plating of a thickness of 10 xcexcm or greater and the conical region with a metallization coating. Additionally, in accord with the invention, a brazing process with preselected parameters is provided.